Puzzles of electron transfer

Major progress has been made to dramatically change our concepts of electron transfer reactions over the last several years by defining the distances, energetics, and even orientation of electron donor and acceptors groups. Despite this progress, several open questions remain. These include the lack of clear understanding of the factors controlling the dependence of the rate on distance. Distance dependence of long distance electron transfer rates is often similar in a variety of experimental situations including rigid glasses, donor and acceptors bound to rigid spacer groups, monolayer assemblies and proteins. But two puzzling cases in which the distance dependence is dramatically weaker have not been understood and interpreted. An even more serious lack of understanding for control of rate by orientation. In some cases strong dependence of rates on solvent polarity have been obtained, but in at least one other case, the rates seem to be independent of polarity. Important energetic quantities such as the free energy change and the solvent reorganization energy are often difficult to obtain and are particularly difficult to predict due to inaccuracies of the dielectric continuum model. This chapter compiles a list of many of these problems, but makes no attempt to suggest the nature of their solutions.